In the field of movement with electric motors, in applications in which reliability and economy are important, asynchronous 2-phase or 3-phase electric motors are widely used. Again for reasons of economy and reliability the motor is supplied through electromechanical switches and phase displacement capacitors. In this manner there is control of the direction of rotation of the motor but its speed is not controlled in any manner.
For example, a typical application is in automation devices for opening of gates. Indeed, this system is employed by nearly all builders of gate openers even though, while being economical and reliable, it displays various drawbacks, mainly in stopping.
In applications in which the motor controls a mechanical gearmotor it is difficult to find a satisfactory compromise between a sudden stop, which is desirable for minimising positioning inaccuracy, and the stopping space in case of emergency stops and a soft stop, desirable for dissipating kinetic energy of the system with the least possible stress on the mechanical parts.
In applications where the motor controls an irreversible transmission or a hydraulic actuator, where it suffices to remove power from the motor to obtain virtually instantaneous stopping of the structure moved, there are severe stresses in the mechanical parts, especially when moving structures of a certain weight. In addition, there are generally used mechanical limit switches which are reached at full speed by the moving structure. In the former case for example, passage bars, etc., this causes unpleasant vibrations and recoil upon striking against the mechanical limit switch.
To obviate these problems it would be necessary to introduce a motor rotation speed control, at least in the acceleration and braking phases. Those skilled in the art know well the different methods of providing such a control. For example, variable number of poles, utilisation of the torque-speed characteristics by reducing the output and control of the power supply voltage, or a.c.-a.c. conversion for control of power supply frequency and voltage. Unfortunately these techniques are relatively costly to apply.
The general purpose of the present invention is to obviate the above mentioned drawbacks by supplying an economical and reliable method and a device in accordance with said method for asynchronous motor speed control and specifically for the movement of gates, passage barriers and the like.